The ability to create remote connections to equipment can provide companies great savings both in time and money. Not only this, but improved customer service can also be offered. It is possible to control, monitor, update software or fault find, irrespective of where the equipment is located, by means of a simple remote connection.
Being able to connect to a remote server whilst working from home or using a laptop PC when travelling makes for much more efficient and flexible working conditions for many support engineers.

Analogue circuits

Connection times between modems are between 15 – 25 seconds.The most common form of remote connection is via the analogue telephone network. The telecommunication circuit is provided by a telecom provider. Dial up, Leased line and multidrop circuits are available.

Dial-up connection – PSTN (Public Switched Telephone Network)

A piece of equipment (e.g. PC, PLC) initiates the PSTN modem so that a connection is established to another PSTN modem by calling its telephone number. The receiving modem answers the call and establishes a carrier. Once the connection has been established between the modems you then have a transparent connection via the serial ports on the modems.

Leased line

A leased line can either be a point-to-point, or a multidrop connection. Multidrop is described in its own section.
Unlike a dial-up connection you have a permanently connected circuit between two points via a 2-/or 4-wire line. This connection can be routed through exchanges or just be a direct cable connection. One modem is configured as the dialling modem and the other the answering modem.
Once the connection has been established at power up data can be continuously transferred.
In the event of a power failure on one of the modems the connection is automatically re-established as soon as the power returns. Modems for leased lines can also be used on private lines.



Multidrop

When it is necessary to connect many devices onto the same circuit multidrop lines can be used. V.23 is an old multidrop standard that was used at 600 or 1200 bit/s on a 2-wire or 4-wire circuits.
The Westermo TD-23 modem permits all rates up to 1200 baud. It is also possible to adjust the transmission power levels and receiver sensitivity of the modem, which means the modem can be used in many applications.
It is possible to have up to 16 drop points at a transmission distance of up to 25 km (15.5 miles).

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Multidrop (Dedicated lines)

Westermo also has multidrop modems for dedicated lines. Products that use the Westermo 10mA current loop as a transfer interface are described in more detail in the “Local Data Communication” section.
The TD-29 is an FSK modem like the TD-23 but requires more bandwidth. Data transfer rates of up to 19.2 kbit/s are possible and up to 10 drop points over a distance of up to 10 km (6.2 miles) can be connected. The TD-29P is specially adapted for equipment using the PROFIBUS protocol.

ISDN
(Integrated Services Digital Network)

ISDN is a digital equivalent to the standard analogue telephone network (PSTN). There are two different standards on the European market, the French VN4 system and DDS1 which applies in the rest of Europe. The two main advantages of ISDN compared to analogue technology is the connection time, which is usually less than 2 seconds, and the data transfer rate of up to 128 kbit/s.

Data Transfer rate

An ISDN-connection is built up of a number of B-channels that transfer data and a D-channel that primarily transfers control signals. The transfer rate on one B-channel is 64 kbit/s and on the D-channel 16 kbit/s. The two most common ISDN services are:

Basic access (BRI Basic Rate Interface), which comprises 2 B-channels and a D-channel. This gives a maximum data rate of 128 kbit/s (2 x 64 kbit/s), which is suitable for users who wish to achieve a higher data transfer rate or for those users who wish to combine telephone, fax and data communications.

Primary access ( PRI Primary Rate Interface) which comprises 30 B-channels and one 64 kbit/s D-channel. The maximum capacity will then be 2 Mbit/s when all B-channels are utilised. This service is used when high data transfer rates are required, for example, for video conferences or for connections between local networks.

Signalling

A digital packet is sent on the D-channel instead of the telecom company activating the ring signal as on the PSTN system. This signal does not interrupt any other call in progress and the connection time is very short. The signal contains information about who is calling, the type of call (voice/data) and the number being dialled.
The connected ISDN equipment then determines how the call should be managed.

Connection

It is possible to connect up to 8 pieces of ISDN equipment to an S0 bus which can have three different bus structures.
A point-to-point connection allows a bus length of approximately 1,000 metres (0.6 miles). A‑short passive bus allows transfer distances of approximately 100–200 metres (330–660 feet) while
the length of an “extended passive bus” is approximately 500 metres (1640 feet).

Modulation

Modem is composite of the word modulation, i.e. signal conversion, and demodulation, which is the regeneration of the original signal.

Examples of modulation principles are:

• Amplitude Modulation
  The amplitude/strength of a carrier wave is varied in accordance with the signal.
• Frequency Modulation
  The frequency of a carrier wave is varied in accordance with the signal.
• Phase Modulation
  The phase of a carrier wave is varied in accordance with the signal.
  
  Phase/amplitude modulation is a combination where more information bits can be transferred per time unit (baud).

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Transfer rate

The data transfer rates you can achieve on a telephone line depends on several factors. The main factor is naturally the quality of the cable, but also the number of exchanges, repeaters and overall distance affect the rate. It is important for modems to match the standards precisely as you may never know the brand of telephone modem at the opposite end of the connection. On leased or multidrop lines we recommend that the same brand is used at each end as there is no true standard for call connection on such lines. There are numerous different standards for telecommunications. Currently the highest possible transfer rate on a modem to modem link is 33.6 kbit. In the V.90 standard it is possible, when connecting to the Internet, to achieve a speed of up to 56 kbit/s, for data traffic from the network to the modem.

Error correction and compression

Data is divided up into blocks and are given a checksum in order to guarantee
communications. If the data transfer is disrupted and the checksum does not correspond the receiver requests the block to be resent. This is called ARQ (Automatic Repeat reQuest) and one of the most common methods for this is V.42.
The technique widely used for compression is described in the V.42 bis standard.
Other methods for error correction and compressed are, for example, MNP 1-10.
(Microcom Networking Protocol)

Transmission distance

Of course on a dial-up connection there are no limitations regarding distance. Here it is the line quality that determines whether a connection can be established or not. Even the speed is dependent on the line quality. Where leased lines are concerned there are several factors that are decisive for the transfer distance.
The main factors are naturally quality and attenuation on the cable, but also the number of exchanges, repeaters and speed affect the transmission distance.

On a leased point-to-point line or a multidrop using the TD-23 distances of up to
25 km (15.5 miles) are possible. Multidrop applications using the TD-29, which can handle line speeds up to 19.2 kbit/s, can achieve transmission distances of up to
10 km (6.2 miles).

GSM
(Global System for Mobile communication)

Connection time between GSM modems is 4-8 seconds (V.110). GSM makes the wireless transfer of voice/text/images between different types of equipment possible, but only if that equipment is within the coverage area of a network operator's base transceiver station. GSM is the digital successor to the older analogue mobile telephone system used in the Eighties.

Initially GSM stood for Groupé Special Mobile, which was the group formed at the beginning of 1980 to draw up a European standard for mobile telephony. Responsibility was then transferred to ETSI, European Telecommunication Standards Institute. The first GSM standard was published in 1989-90. Today this standard is
recognized on most continents, and GSM now stands for Global System for Mobile communications. On account of this it is possible to use your GSM equipment on different operator's networks and in different countries (Roaming).

The number of users of GSM equipment has, after standardisation, increased rapidly, primarily with voice services, but a major increase is now also occurring within industrial applications, in particular M2M communication, (Machine to Machine).
This can be to transfer data or alarms from a number of slave units to a master system or the transfer of data from or between parking meters and other similar equipment. This area of application is almost unlimited and there will be a rapid development of different types of GSM equipment to cover future needs.

One important prerequisite to achieve functional GSM data communication is to have a contract with a network operator, SIM card and data number to allow data transfer. If you only intend to send SMS-messages then you do not need a special data number.

Structure of a GSM network

A GSM network can be divided up into three different parts:

• Mobile equipment (MS, Mobile Station)
• Base transceiver station (RAN, Radio Access Network)
• Core network, which is the connection to external networks for example,
  ISDN and PSTN networks (CN, Core Network)

Cellular structure

A GSM network is built up of a number of base transceiver stations and the area it covers is called a cell.
The coverage area of the base transceiver stations varies from approximately 100 m (300 feet) up to 30 kilometres (18.6 miles) in radius depending on the output power. Each base transceiver station can handle a finite number of users, which is why, in an urbanized area, base transceiver
stations are located close together to be able to handle the greater call density.

Radio communication (frequencies)

The development within mobile communication has resulted in the need to use multiple frequencies to satisfy demand. Today there are five standardised frequency ranges: GSM400, GSM850, GSM900, GSM1800 and GSM1900. GSM850 and GSM1900 are used in countries where the other frequency ranges are already in use. This is the case in USA and parts of South America and Asia, but GSM900 and GSM1800 MHz are the most globally used frequencies.

Services

GSM offers a number of services, these are briefly described below:

Voice
Voice is naturally the service that is used the most and the number of users rises continuously. Development is ongoing where the aim is to minimise bandwidth usage while maintaining call quality and in doing so increase the possible number of users with the same number of base transceiver stations.

SMS
SMS allows the sending of text messages of up to 160 characters to any other equipment that can handle SMS. The most common application area today is between mobile telephones.

Data
CSD (Circuit Switched Data) makes it possible to transfer serial data at speeds from 300 up to 14400 bit/s.

There are two different function modes to transfer serial data across the GSM network, non transparent mode and transparent mode. An error correction protocol called RLP, Radio Link Protocol is used in non transparent mode. This ensures the secure and correct transfer of data, but also results in a data delay. An error correction protocol is not used in transparent mode and consequently there is no transmission delay.

GPRS

GPRS is an extension of GSM to allow the handling of packet switching data traffic. Each channel not in use for voice traffic can be utilised for packet switching data traffic. Packets from several different users can be mixed within the same channel, which results in efficient sharing of available network resources.
GPRS also permits higher transfer rates as it can use multiple channels within
GSM’s radio capacity. In theory rates of up to 171.2 kbit/s can be achieved, however, transfer rates of between 20–50 kbit/s are more common (compared to HSCSD, High Speed Circuit Switched Data, which offers rates from 9.6–43.2 kbit/s which some operators also offer for circuit switched GSM traffic). However, transfer rates are dependent on several factors such as operator, terminal, number of users in the same cell, distance to the base transceiver station (resending), when equipment is on the move (handing over between base transceiver stations also lowers the transfer rate), etc.

Another advantage of GPRS is that the equipment is always connected to the network and charges are made according to a volume tariff. That is to say, you only pay for the data sent and received by the equipment not the time connected.

FAX

The following Fax standards are available on GSM networks: Class 1 and Class 2.

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